Actually, I don't understand why the decrement operator is not defined on a bool. However, if I were the king of X3J16, I'd not support a wrap-around approach to the increment or decrement operators. I'd support:
\nfalse++ == true\nfalse-- == false\ntrue--  == false\ntrue++  == true\n

(with the pre-increment and pre-decrement operators working the same way.)

I'd also not support implicit conversions to or from bool; you'd have to explicitly cast a non-bool to a bool or vice versa. (That should mitigate Todd's objections a teensy bit....)